This page briefly explains how to use various tools which enhance your privacy, anonymity and overall security. The guides are written in an easy to understand, step-by-step manner. The difficulty & time required for most of them don't provide any reason to not secure your communications and blurring your digital traces.

This page has grown and is hard to navigate in. Recommended re-arrangement:

Move each guide under separate article, not headline

People attending crypto parties carry different devices with different operating systems. Therefore, do not arrange stuff under Windows, OSX, iOS, Android etc, but instead under topics, and then explain how to do that for each system. This is because general, cross-platform introduction to each technology (e.g. what is E2EE messaging or FDE) is usually required, and having a copy of what is is FDE for each OS creates pointless redundancy.

Make this a landing page with short explanation of each tech and add link to actual article(s).

Security warning

Note, however, that security is a process, not a tool. You need at least basic understanding to assess the degree of security or anonymity a tool can give you. That said, treat it like a game. The worst thing which can happen, if you use these tools for your everyday business, is that you are just as unsecure, unencrypted or in the open as you would be anyway.

The use of security sensitive activity (which is both prohibited and can be persecuted by society and/or government) without deep understanding is strongly discouraged.

Web Browsing

To get an idea of what web browsing actually is, read the chapter Understanding Browsing of the CryptoParty Handbook. In brief:

When you visit a website you give away information about yourself to the site owner, unless precautions are taken.

Your browsing on the Internet may be tracked by the sites you visit and partners of those sites.

Visiting a website on the Internet is never a direct connection. Many computers, owned by many different people are involved. Secure connections ensure that your browsing can not be read in between you and the server.

What you search for is of great interest to search providers (mostly for targeted advertising).

Then you can see what you just learned by facing a virtual mirror to yourself on

Browser

Firefox is an open source web browser that respects your privacy. If you're not using it already you should do so from now on. It's as good as Chrome, fast and the most extendable browser with most AddOns. It's available for Windows, Mac & Linux.

Tor Browser

Most possibly the best option you can and will ever have if you like your privacy, freedom and anonimity in your hands, and not in the hands of trackers, companies -like your personal ISP-, governmental systems and programs, and other potential snoopers, where they don't belong.

Before browsing .onion sites make sure to click on the in the upper left of the browser and choose “Forbid Scripts Globally”. This prevents JavaScript from leaking potentionally personally identifiable information - disable for individual sites if needed.

You should also set the security level to High and lower it only if it has major effect on your browsing experience. The Security setting can be found under the Tor logo in navigation bar.

Browser Plugins

HTTPS Everywhere

HTTPS Everywhere has a big list of websites that support encrypted connections, and whenever you connect to them silently switches to the encrypted variant. That little “s” in the URL is what it is about.

Scripting

Identifiable Browser configurations

The only serious attempt to thwart browser fingerprinting is the Tor Browser.

Request Policy

Advanced.

Request Policy is an open source Firefox extension to control cross-site requests.

Certificate Patrol

Your browser trusts many certification authorities and intermediate sub-authorities quietly, every time you enter an HTTPS web site. The Firefox AddOn Certificate Patrol reveals when certificates are updated, so you can ensure it was a legitimate change.

Web search

Another thing you might do often on the web is use Google to search things. There are plenty of alternatives to Google who all state that they keep minimal or no IP logs, but blind trust isn't as good as using Tor Browser to actively hide your IP. Most popular ones are:

In Firefox you can do the same for startpage but might have problems with encrypted.google in recent versions of firefox. Go to the page you intend to make your search engine and select the logo to the right of your search bar (top right), and select Add “[searchEngineName]” to change search engines.

General Tips

Regularly run CCleaner (Windows & Mac) or BleachBit (Windows & Linux) for deleting cookies and various other junk.

Check the privacy settings of websites. For example if you have a google account you can deactivate the logging of your searches and the personalized advertisements. Log in to your account (android phones come with google accounts) and change various settings on the dashboard

Own Website

Make your website available via HTTPS, or even better, redirect unencrypted connection attempts to the encrypted version. First follow these instructions for getting the certificate then install it as in the appropiate tutorial here. Secure Sockets Layer provides an encrypted connection between the client and the server/certificate holder.

Closing Unused Ports (Linux)

Check open ports.

From the command line, you can see which ports are open on which interface by typing:

sudo lsof -i -P | grep LISTEN

* means it is listened on all interfaces (reachable from the outside)

localhost means the ports are only opened locally (not reachable from the outside).

Services can be removed, disabled, or configured to only listen locally.

For more control over your email, you have to either run your own mail server or have a good personal trust relationship with the provider.
There are some ways to get a new email account with a bit more privacy:

Ask a geek/nerd friend

Pay for the service (instead of paying with your data)

Combine the above (actually the very best option)

Use email from a non-profit organization (and donate money if you can)

Crypto! (GPG-Encryption)

As you may know, your email goes through the data traffic like a postcard in snailmail: Everyone can read it! So, like snailmail, it would make sense to put your emails in a closed envelope. One possible envelope is called GPG.
The Pretty Good Privacy software was originally written by Phil Zimmermann, and is now owned by Symantec. The means of encryption defined by that software are also called PGP - these standarts are now freely available as OpenPGP which derived from the original PGP.
The GPG software is an independent implementation of the OpenPGP standards, so you can use it to exchange encrypted messages with people using other OpenPGP implementations (and Symantec's PGP).

Warning!

While email encryption is still mostly secure, the nature of PGP messages has two inherent problems.

Lack of forward secrecy: PGP uses long term decryption keys that never change. If at any point in future your device is stolen, accessed or hacked, all past messages recorded by powerful attackers can be decrypted, even if you have deleted messages from your own devices.

Lack of deniability: In PGP, the authorship of messages is verified with what are called digital signatures. These digital signatures can only be created by the sender, and any message you send can be proven to have been written by you.

These problems have since been solved in modern end-to-end encrypted messaging systems like OTR, Signal, etc. that are also easier to use (see below). Therefore, unless you absolutely have to, it is advised to always use modern messaging applications instead of PGP.

Understand

For your first time, you should get a basic understanding at least of the concept of asymmetric encryption (often called public key encryption). Please watch one of those videos before you begin using it:

3 minutes: BBC science presenter Dr Yan Wong explains (without mathematics) the principle of how Alice and Bob can use “digital padlocks” to protect their messages from being read by Ed the eavesdropper

2. Install GnuPG

3. Plugin Enigmail

Enigmail is a plugin for Thunderbird that brings thunderbird and GnuPG together.
Find the add-on manager in your Thunderbird (upper right side menu) and install enigmail there. On Linux, install it via your software manager. The package is usually called enigmail.

4. Passphrase

Now you want to give yourself some time to think about a nice passphrase and making sure you remember it.

5. Generate Keypair

Click OpenPGP in the Thunderbird menu and

choose OpenPGP Setup Assistant or … Wizard (depending on version).

Follow the instructions. When not sure, the default value is usually safe.

Afterwards, it will ask you if you want to make a revocation certificate. Do so, and store it on a safe medium (that is either a print-out or a CD you burn it to and then put away in a safe place).
If you have already generated a keypair or want to follow instructions like the ones given keypair or want to follow instructions like the ones given by Alex Cabal, you should run the Setup Assistant anyway and then choose the already generated keypair at the appropriate step of the wizard. For a more detailed description of the mechanism of public-key encryption, please refer to keypair at the appropriate step of the wizard. For a more detailed description of the mechanism of public-key encryption, please refer to The GNU Privacy Handbook.

6. Publish Public Key

If you now think “WTF publish my KEY!!11!!!” please watch the above videos again :P
Link it on your website/message it your friend and/or get it up a keyserver such as this one

To get a copy of a public key on Linux with GNUPG run the following command:

gpg --export --armor <your GPG ID>

this will generate output starting with '—–BEGIN PGP PUBLIC KEY BLOCK—–' and ending with '—–END PGP PUBLIC KEY BLOCK—–'. '–armor' makes the key read- and printable.

7. Get your recipient's Public Key

If your intended recipient doesn't already use PGP get him to work through this tutorial first. Then get his public key which you can find on a keyserver/website if he doesn't message you it directly. On Linux using GNUPG, your intended recipient should follow the process in step 6 and output it to a file, once you've received this file use the command:

gpg --import /path/to/file.key

The key will now be available to be accessed through GNUPG and thus through Enigmail or other programs that utilise GNUPG.

From the command line, you can see your local collection of keys by typing:

gpg -k

To find a particular key, type:

gpg -k <part of name/email/key ID>

To display or search keys in Thunderbird/Enigmail:

Choose “OpenPGP” in the Thunderbird menu

Choose “Key management”

Type part of a name or email in the search box, or check “Display All Keys by Default”

8. Write your first encrypted email

Only encrypt plain text and note that subject lines are not encrypted.

You can use the command line to encrypt a file or a message:

gpg -ase -r <recipient's key ID> -r <your key ID> <input file name>

This will produce a file (ending in .asc) that you can attach or paste into an email.

Depending on how Thunderbird is set up, it may give you a list of keys to choose from at this point, or it may select keys automatically based on email addresses (This behavior is configurable: OpenPGP → Preferences → Key Selection.). If you see the list of keys, make sure the recipient's key and your key are checked, and click OK.

To decrypt a message from the command line, save the encrypted message to a file, and type:

gpg <encrypted file name>

To decrypt mail with Thunderbird/Enigmail:

Click on the messge.

After a moment, the passphrase entry box should appear; enter your passphrase.

To verify a signature:

If the message was signed, there should be a “Good signature” message (visible in the output of the command-line client, or a green bar above the sender information in Thunderbird). If there is a “signature verification failed” message instead, it could mean that the message was tampered with, or it could just mean that you don't have the sender's public key.

Start up Outlook and make a new email. In the right upper panel you can encrypt (and also sign) your email.

Before you send an encrypted email you need your recipients public key block. For testing purposes you can create another account (with a trashmail address) which you delete later. Otherwise you find such keys on websites/directory services or elsewise.

Once you have the recipients public key copy it (from & including “—–BEGIN PGP PUBLIC KEY BLOCK—–” until end), open up GPA and simply press ctrl+v (paste).

Now make a new email in outlook and fill in the recipients' email address.

Enter whatever text you want to send. And then click “encrypt” in the right upper corner.

Make sure your recipient has your public key as well.

To decrypt a message you received double click the email and then coose “decrypt” in the right upper corner and enter your password.

More Information

Maybe it wasn't that easy for you to do it, or maybe you want to know more. In either case, please have a look at the following links to some guides and more information:

GPG-Encryption beyond Email (GPA)

If you'd like to use GPG (for an explanation of GPG please see the upper section "Crypto! (GPG-Encryption)") for other purposes besides just Email the video-tutorial beneath might help you out.
If you're using Windows simply install the The GNU Privacy Assistant (GPA) of the GPG4Win-Installer and use the Clipboard to encrypt and decrypt messages by hand. Tutorial:

Deniability: The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you. However, during a conversation, your correspondent is assured the messages he sees are authentic and unmodified.

Forward secrecy: If you lose control of your private keys, no previous conversation is compromised (assuming control of log files was not lost at the same time).

How to use

Signal protocol

Signal-protocol is a modernized version of OTR-protocol that is designed to work in asynchronous environments such as on smartphones. This is because on smartphones apps open and close so frequently, OTR-sessions (that need to be established for each time they're used) become inconvenient.

Other

Retroshare lets you securely chat and share files with your friends and family, using a web-of-trust to authenticate peers and OpenSSL to encrypt all communication. It provides filesharing, chat, messages, forums and channels.

TorChat is a peer to peer instant messenger with a completely decentralized design, built on top of Tor' s hidden services, giving you extremely strong anonymity while being very easy to use without the need to install or configure anything.

I2P Messenger is an end-to-end encrypted serverless communication application over I2P. It supports file transfer and has a search for other users.

BitMessage is a P2P communications protocol used to send encrypted messages to another person or to many subscribers. It is decentralized and trustless, meaning that you need-not inherently trust any entities like root certificate authorities. It uses strong authentication which means that the sender of a message cannot be spoofed, and it aims to hide “non-content” data, like the sender and receiver of messages, from passive eavesdroppers like those running warrantless wiretapping programs. Tutorial for setting up and using Bitmessage – an encrypted communications platform based on Bitcoin

Jitsi may request non-secure information during encrypted chat if you paste a link into it. This can be disabled in “Preferences/Options > Chat > Enable Image/Video replacement”

CSipSimple is an open source android app for end-to-end encrypted VoIP calls.

Get a free SIP account for Jitsi and/or CSipSimple with The Guardian Project’s Ostel service.

Darknet

A darknet is an internet or private network, where information and content are shared by darknet participants anonymously. More accurately all of them share being anonymous overlay networks.

Tor Hidden Services

Tor can also provide anonymity to websites and other servers. Servers configured to receive inbound connections only through Tor are called hidden services. Rather than revealing a server's IP address (and thus its network location), an hidden service is accessed through its .onion address. The Tor network understands these addresses and can route data to and from hidden services, while preserving the anonymity of both parties.

I2P

I2P is a secure, anonymous network resistant to censorship and monitoring and both distributed and dynamic, with no trusted parties. It offers a range of services by default (including an active IRC Chat) and with full support for streaming, anonymous file sharing (BitTorrent), webserving, mail and more. See the comparison between Tor and I2P

Step 2

On the left panel you will see bandwidth of 96KBps and 40KBps for the In and Out speeds. Your most likely have an Internet speed far greater than this. Therefore, you should raise the speeds significantly.

When installed click the FoxyProxy logo next to the URL bar. And then change “Select Mode:” to “Use proxies based on their pre-defined patterns and priorities”

Click “Add a new proxy” and on the “General” tab, make sure “Enabled” is checked. Also give it a name like “I2P” there.

One the “Proxy Details” tab, select “Manual Proxy Configuration” and enter “localhost” in the “Host or IP Address” field and “4444″ in the port field.

On the “URL Patterns” tab, click “Add New Pattern”, make sure “Enabled” is checked and “Whitelist” and “Wildcards” are selected. Give it a Pattern Name (ie. “I2P”) and in the “URL Pattern” field, enter “*.i2p/*”

Press Ok twice & close. Firefox will now send all .i2p requests through the local proxy. You can now access the “eepsites” hosted within I2P.

Alternatively (and recommended for optimal security) you can create another Firefox profile (ie “I2P”) go to Extras→Options→Network→Connection Settings→check Manual Proxy Configuration and then enter the following:

HTTP-Proxy: 127.0.0.1 Port: 4444

Click OK. You can also run 2 firefox instances at the same time using this neat batch

Enter about:config and confirm that you're being careful. Search for the following entries and set them all to false:

Freenet

Freenet is a peer-to-peer platform for censorship-resistant communication. It is more or less a decentralized distributed data storage. Freenet works by storing small encrypted snippets of content distributed on the computers of its users and connecting only through intermediate computers which pass on requests for content and sending them back without knowing the contents of the full file, similar to how routers on the Internet route packets without knowing anything about files—except with caching, a layer of strong encryption, and without reliance on centralized structures. This allows users to publish anonymously or retrieve various kinds of information. So called “freesites” allow you to browse such content. Other types of usage include chat, email & microblogging.

Retroshare

RetroShare is free software for encrypted, serverless email, Instant messaging, BBS and filesharing based on a friend-to-friend network built on GPG. Unlike most P2P networks where your computer will connect to the network and share information with a huge number of unknown peers, RetroShare will only connect to other peers that you have explicitly allowed into your network, and all communications are private.

Communication services in RetroShare:

Private chat with friends

Private or public chat lobbies, that allow chatting with friends and friends of friends

Messages to friends

Forums

Voice over IP

All you need to do is install the software and generate a PGP/GPG key, which will be used to encrypt and decrypt your network traffic. The hard part is getting at least 5 of your friends to also install the software and to share their public keys with you. Once that is done, you have your very own DarkNet.

Please add info for “The degree of anonymity can still be improved by deactivating the DHT and IP/certificate exchange services”

Meshnet

Advanced. A meshnet is a decentralized peer-to-peer network, with user-controlled physical links (usually wireless). The most popular meshnet refers to the transitional CJDNS Internet overlay network currently known as Hyperboria.

Other

Anonymous Upload & Download of Youtube-Videos

Videos from Youtube have unique metadata embedded into them via our friends at Google (on a per download basis). If that same file is seen elsewhere Google can check their logs to see when that file was downloaded and everything your computer sent, such as your IP address, user-agent and other fingerprinting info.

When using youtube-dl: Make sure to use the false user-agent that Tor Browser uses. Youtube-dl uses your real computer user-agent otherwise, which is not good for privacy.

If you plan to reupload or share the video and wish for google to not know which of the downloaders is uploading the file do the following from a Linux terminal:

$ ffmpeg -i originalvideo.mp4 -acodec copy -vcodec copy newvideo.mp4

That will strip the video to only the video and audio (removing the metadata). You can verify this by downloading the same video twice and checking the sha256sum's against each other. After you strip the video and audio you can see the two different sha256sum's have become the same.

DNS

The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates easily memorized domain names to the numerical IP addresses needed for the purpose of locating computer services and devices worldwide. An oft-used analogy to explain the Domain Name System is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the domain name www.example.com translates to the addresses 192.0.43.10 (IPv4) and 2001:500:88:200::10 (IPv6)….which you probably can't remember as good as a name such as “example.com”. Here is a video explaining DNS

OpenNIC is an alternative DNS root which lists itself as an alternative to ICANN and its registries. By using it your connection to the Internet can't get censored by your DNS server. It also allows you to use DNS servers which don't run logs improving your anonymity.

Currency

To anonymize your bitcoins further you can use the BitcoinFog Or Helix laundering service over Tor

File Deletion

If you want to delete files on your PC the normal way, they can be easily restored with tools freely available on the Internet (such as Recuva). Because of this you might want to make sure to truly delete files in certain circumstances (ie if you want to sell your PC).

Warning

Right now, there is no secure way to delete files from flash memory. This includes usb sticks, memory cards and solid state hard disks (SSDs). The only responsible way to prevent theft of data on these media is full disk encryption.

Windows

DBAN is a self-contained boot disk that automatically deletes the contents of any hard disk that it can detect.
This method can help prevent identity theft before recycling a computer. DBAN prevents all known techniques of hard disk forensic analysis. Warning to make this perfectly clear: it will erase all data on all hard drives it detects (including external ones(!))“.

With Ccleaner you can do the same for partitions, drives as well as seemingly “free space” [which in reality consists of restorable data] on Windows & Mac. For this go to Tools→Drive Wiper.

Linux

If you want to erase a hard disk (now, because everything is overwritten, this works with flash memory, too), you can simply do so by finding out the file representation of the disk, e.g. /dev/sdx and then executing

dd if=/dev/urandom of=/dev/sdx

as root/superuser. This command is irrevocable, so please double-check before executing it! \\To find a list of current 'block devices' you can use the 'lsblk' program, this will provide a list of the current available block devices by their name. Please note that if you want to properly purge the data you want to overwrite the root device, ie /dev/sda rather than /dev/sda1. as /dev/sda1 is a partition within the block device.

BleachBit provides a means of clearing common caches and other meta information left behind by applications and also includes a 'Free disk space' option, which will attempt to obscure the contents of free disk space by overwriting available disk space with random data (it creates a file, and lets it grow till it consumes all free space) and a 'Memory' option which will do the same for RAM and Swap.

THC Secure Delete provides a set of tools for surely erasing files, swap and memory. srm does secure deletion of files.
sfill does a secure overwriting of the unused diskspace on the harddisk.
sswap does a secure overwriting and cleaning of the swap filesystem. (note that sswap was only tested on linux so far. you must unmount your swap first!)
smem does a secure overwriting of unused memory (RAM) To install the tools on ubuntu issue the command:

sudo apt-get install secure-delete

Mac

Permanent Eraser provides an even stronger level of security by implementing the Gutmann Method. This utility overwrites your data thirty-five times, scrambles the original file name, and truncates the file size to nothing before Permanent Eraser finally unlinks it from the system. Once your data has been erased, it can no longer be read through traditional means.

Photos & Videos

Photo EXIF Data Removal

EXIF (Exchangeable Image File) data is a record of what camera settings were used to take a photograph. This data is recorded into the actual image file. Therefore each photograph has its own unique data. EXIF data stores information like camera model, exposure, and sometimes even GPS-data. While there are many image-hosting services such as imgur.com that strip away the exif data most sites keep it, leaking private information ie for grab to the NSA's XKeyscore program which is planned to mine for the exif data of all pictures getting uploaded.

Other

ObscuraCam is a secure camera app for android phones that can obscure (ie for face blurring), encrypt or destroy pixels within an image.

Virtual Machines & Live Disc/USB

The Amnesic Incognito Live System or Tails is a Debian-based Linux distribution aimed at preserving privacy and anonymity. All its outgoing connections are forced to go through Tor, and direct (non-anonymous) connections are blocked. The OS is designed to be booted as a live CD or USB, and leaves no trace on the machine unless explicitly told to do so.

Alternatives to Tails such as Liberté Linux can be found here. The following tutorials also pretty much apply to them as well.

Virtual Machine

A virtual machine is a software based, fictive computer. Virtual machines may be based on specifications of a hypothetical computer or emulate the computer architecture and functions of a real world computer.

Live Disc/USB

A live disc is a complete bootable computer operating system which runs in the computer's memory, rather than loading from the hard disk drive. It allows users to experience and evaluate an operating system without installing it or making any changes to the existing operating system on the computer. Live USBs are closely related to live discs, but sometimes have the ability to persistently save settings and permanently install software packages back onto the USB device.

Operating system

If you (keep) using Windows xp-AntiSpy lets you disable some built-in update and authentication ‘features’ in Windows 7 that are calling home. For Windows 10.

VPN

A Virtual Private Network (VPN), is a private network of computers within a public network (the internet). When you connect to a VPN, the computer acts as if it’s on the same local network as the VPN. All your network traffic is sent over a secure connection to the VPN. Unlike a Proxy, a VPN service provider encrypts all of your traffic, replacing your ISP and routing ALL traffic through the VPN server, including all programs and applications while being faster as each client gets dedicated resources (a single proxy often has thousands of users).

Android

General

Antivirus

You should definitely have an anti-virus software running on your device.

Root

Many apps require root-access to your phone. Gaining such isn't that hard to do: just google your device name and firmware (both to be found in the settings under “info to device”​) + “root tutorial”​ as it's different for each device and firmware-version. However there are also multiple reasons of security to not root your device.

Orbot is a free proxy application that empowers other applications to use the Internet more securely. Orbot uses Tor to encrypt your Internet traffic and then hides it by bouncing through a series of computers around the world.

History Eraser allows you to delete your search history and various other things (just like Ccleaner/BleachBit for your mobile). It also guides you to some settings that ought to be changed or switched off such as google data syncing. There also is CCleaner for Android.

Notes

iOS

iOS is a proprietary operating system whose source code is not available for auditing by third parties. You should entrust neither your communications nor your data to a closed source device (better use android or any of these alternatives).

Web Browsing

Chat

Disc Encryption

VeraCrypt

VeraCrypt is an on-the-fly disk encryption system and the successor of the discontinued TrueCrypt. The software is freely available, runs on multiple operating systems, and is very easy to learn how to use. VeraCrypt also plays nicely with dual-boot systems (such as Windows and Linux). VeraCrypt options include either full disk encryption or the creation of cryptographic container files, which mount as additional drive volumes.

VeraCrypt can also be used to encrypt USB flash memory sticks or digital camera or mobile phone memory cards. The caveat is that it is almost impossible to guarantee to securely wipe or overwrite the data from these devices due to their wear leveling algorithms. Therefore you should use a fresh USB device to re-encrypt the data with a new secret key. VeraCrypt also includes a few options which theoretically provide plausible deniability to the user.

FileVault

Since version 10.6 of Mac OS X, Apple has offered users the ability to encrypt the home directory of their system. And from 10.7 onwards, Full Disk Encryption has been an option (technically referred to as FileVault 2). Enabling FileVault requires the user to have admin privileges on the computer, and will prompt the user to restart. At the next boot, as soon as the user logs in, FileVault will start doing online encryption of the main system drive. Other drives connected to the computer can also be encrypted by selecting them in Finder and choosing “Encrypt” from the File menu.

When enabling FileVault, in addition to admin users being able to unlock the drive at login, a Recovery Key is also generated, with the option of escrowing this key with Apple. If you choose to do that, you'll have to provide various additional security questions/answers along with your Apple ID. Given the ease of use of FileVault, it should be almost the first thing you should enable on setting up a new Mac. Unfortunately, it doesn't currently work on RAID drives.

Integrity Checks

In order to check that you're actually using the right program and not a fake or modified/backdoor'ed one it's recommended to do integrity checks (for things such as the Tor Browser Bundle at least). A 'hash' is a unique number generated using a published algorithm on a particular file. For example, if I have file1.txt, which has no text in it, and I run it through a hashing algorithm, I will get mathematical_value_1. If I then add text to the file, it has now changed and if I hash it again I will get a different result, mathematical_value_2.

sha1sum and md5sum are included in most Unix/Linux based operating systems (including MacOSX) → Go to 'Terminal' in Applications→Utilities, navigate to the file you wish to use and type 'md5sum ' where 'filename' is the filename, to get the md5sum.